The invention concerns a method for the production of synthesis gas, in which the crude gas obtained through partial oxidation (gasification) of carbon-containing material is subjected to a catalytic conversion in a conversion arrangement which displays a moistening-demoistening system connected by means of a combined water circulation.
It is known to proceed with the generation of synthesis gas from carbon-containing material (coal/petroleum), from which initially a crude gas is obtained by partial oxidation (gasification), which thereafter can be worked up through suitable further treatment into synthesis gas for the methanol or ammonia or oxygen syntheses. The generated synthesis gas is composed, depending upon its further applicability, either predominantly of hydrogen or of a mixture of hydrogen, carbon monoxide and/or nitrogen. In most cases a reduction of the carbon monoxide content and an increase of the hydrogen content of the crude gas yielded by the partial oxidation (gasification) is necessary. The crude gas is for that reason in these cases initially subjected to a cooling and dusting as well as, if necessary, a desulfurization, made necessary in this case on account of the sensitivity to sulfur of the catalyst used for the conversion. The catalytic conversion follows thereby according to the reaction EQU CO+H.sub.2 O.revreaction.H.sub.2 +CO.sub.2.
This reaction is known to be exothermic and requires for adjustment of the equilibrium an addition of steam to the gas to be converted.
In order to save steam there are accordingly already in use conversion arrangements, which are provided with a moistening-demoistening system connected by means of a combined water circulation. The manner of operation of this apparatus may be sketched in briefly as follows: The gas to be converted is initially led from below into the moistener equipped with inserts as usually used in gas washing towers in which it is sprayed counter-current to suitably heated water introduced from above, whereby the gas at least partially undergoes the steam saturation necessary for the conversion. After the catalytic conversion, which can be introduced in one or more stages, the hot gas is ushered from below into the so-called demoistener, which is likewise equipped with inserts. In these, the partial condensation of surplus, non-converted steam from the converted gas follows through spraying with appropriately cooled water. For this purpose the water running out from the moistener, after appropriate cooling, is introduced into the demoistener. This introduced water is then, together with the water condensed by the converted gas, drawn off from the sump of the demoistener and pumped back in circulation to the moistener. After the necessary re-heating, it is used there again as spray water available for the gas to be converted.
Through use of such a moistener-demoistener system the necessary amount of fresh steam for the conversion is reduced. In the interest of a further reduction of the fresh steam requirement and a further optimization of the heat economy, a synthesis method must, however, take into consideration the entire process of gas treatment including the synthesis stage, and as far as possible, the there yielded process waste heat must be used by means of recovery, e.g. in the form of high-pressure steam. With the conversion of partial oxidation gas, temperatures occur which lie in the range of 400.degree.-500.degree. C. and offer the possibility of high-pressure steam production. With the subsequent synthesis, for example of ammonia or methanol, useful waste heat in the temperature range of about 200.degree.-330.degree. C. is produced by the exothermic formation reaction. This waste heat is excellently useful for the conversion of the crude synthesis gas, since the temperature of the water circuit in the moistener-demoistener system indeed, according to operating pressure, lies somewhere between 150.degree. and 250.degree. C.
From German Offenlegungsschrift DE-OS 29 22 293 a method is already known for the production of methanol from synthesis gas, in which the waste heat in the gas stream leaving the synthesis reactor is used in indirect heat exchange for the heating-up of the circulation water flowing from the demoistener to the moistener. Thorough investigation by the inventors has however shown that with use of such a circuit considerable difficulties can appear with the conversion of the crude synthesis gas. Namely, the steam content in the gas obtainable with this circuit before the conversion is not sufficient for the smooth execution of this reaction. With the catalysts known today there have been observed, to the contrary, undesirable secondary reactions such as the formation of soot and/or methanization.